Change field of a node in a group
You can directly access node group data from 

node_group = bpy.data.node_groups['NodeGroup']


The node group name is the one chosen in the group node



Each node group then has it's own nodes -

node_group.nodes


You can find a node groups nodes from a material (don't confuse node name with node group name)

bpy.data.material['Material'].node_tree.nodes['Group'].node_tree.nodes




You can get the node group name from the materials group node in node_tree.name

for n in mat.node_tree.nodes:
    if n.type == 'GROUP':
        node_group_name = n.node_tree.name


See this answer for an example of creating a node group using python.

Once you have the group nodes you can access them the same as normal material nodes.

node_group.nodes['Diffuse BSDF'].inputs['Color'].default_value = (1.0,0.0,0.0,1.0)